Especially when switching solutions from hypoosmol to "normal ACSF" it
is very important to slowly exchange solution. Ideally, a peristaltic
pump will be pumping regular ACSF into the bath containing the finished
slices while remove excess solution at the same time. A good rate should
be 2 to 5 mL per minute.
If you use ascorbic acid or its salt form sodium ascorbate shouldn't
matter, as long as you adjust the pH properly. In both cases the two
forms will be present in an equilibrium that will depend on the pH of
your solution (according to the Henderson-Hasselbalch equation).
h2h, Christian